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Measuring Pavement Thickness Nondestructively using Pulse Induction Technology Current Agency Practices TRB Session 1400 Quality Assurance in Construction and Smart Technology Jagan Gudimettla, P.E. ATI Inc (Consultant to FHWA) Office of Preconstruction, Construction, and Pavements
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Acknowledgments Todd Hanson (Iowa DOT) Kevin Jones (Iowa DOT)
Chad Hayes (Wisconsin DOT) Robert Golish (Minnesota DOT) Gregory Johnson (Minnesota DOT) Richard Howell (Minnesota DOT)
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Outline Pavement Thickness Pulse Induction Method
State DOTs Experiences Concrete Pavement Thickness Asphalt Pavement Thickness Current State of Practice Summary
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Pavement Thickness Significantly influences the life of pavement
Important quality assurance activity for agencies Methods to measure pavement thickness Probing (Quality Control) Coring (Most Prevalent) (Acceptance) Non-destructive methods Impact Echo Ground Penetrating Radar (GPR) Ultrasonic Pulse Echo (MIRA) Pulse Induction Technology (MIT Scan T2/MIT Scan T3)
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Traditional Tests Destructive Expensive Time Consuming - core
- inspect - handle - measure - patch core holes Probing (Quality Control) Coring (Acceptance)
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Pulse Induction Technology
Step 2 Step 3 Step 1 Find the target and measure thickness Place the target Pave over it Quality Control and Agency Acceptance AASHTO Test Method (AASHTO T ) ASTM Test Method in the works
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Pulse Induction Technology
Advantages Easy to use High accuracy Non-destructive Almost real time Rapid measurement Significant cost savings Data storage (text files) Independent of the base material Calibration with core not required
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Pulse Induction Technology
Accuracy FHWA Mobile Concrete Trailer (MCT) Data
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Pulse Induction Technology
Limitations Only for new construction Measurements 2-3 feet away from steel GPS enabled Hardware enhancements Software enhancements MIT Scan T2 MIT Scan T3
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Pulse Induction Technology
Metal Targets or Reflectors 4.7" 2.7" 4.7" 11.8" 11.8" 14“ Asphalt Concrete
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Field Implementation FHWA Mobile Concrete Trailer (MCT) Since 2007
Field Visits Equipment Loan
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Iowa DOT Timeline Technology introduced (Spring 2008)
Equipment Loan (Fall 2008) Evaluated the technology on two projects (2009) Trial Specification – four projects (2010) Full Specification (2011) Purchased several devices for each of their 6 districts Worked with MCT staff to develop an AASHTO Test Method ( )
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Question If the MIT Scan T2 needs metal targets… Do we lose the element of random sampling (surprise)?
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Iowa DOT Procedure Place targets every 200 ft randomly 4 or 8 foot right or left of centerline (avoid steel) One nail hole in the middle of the target
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Iowa DOT Procedure Random number (~50 to 65%) of targets are randomly selected The MIT Scan T2/T3 is used to find the location of the target Test by slow walking speed. Test 3 times. Results need to be within 3 mm, if not, 2 additional readings are taken
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Iowa DOT Old Spec New Spec Core 1 random location every 2000 sq. yds.
Four metal targets every ~2000 sq. yds. Two MIT Scan measurements every ~2000 sq. yds.
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Iowa DOT Issues with Granular Subbase 9 point measurement
Core off target, core on target 9 point measurement Concrete migrates into the aggregate base Core test results are slightly thicker
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Iowa DOT Thickness Incentive Thickness index =
(Average Thickness – Std Dev.) – Design Thickness Unit price adjusted up to 103% - or less Correction for type of base Type of Base, Subbase, Subgrade just below the concrete Adjustment to Design Thickness for Incentive Calculation Natural Subgrade or Soil Aggregate Subbase Design Thickness HMA Base, PCC Base, or Asphalt or Cement treated Base Modified Subbase or Special Backfill Design Thickness minus 0.25 inches Granular Subbase Design Thickness minus 0.35 inches
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Iowa DOT Currently have 12 MIT Scan T2 and 4 MIT Scan T3
Agency tests verification and Materials checks Independent Assurance with different gage
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Minnesota DOT Timeline Since 2013
Dowel baskets and tie steel Evaluated for thickness in 2015 and 2016: 9 pilot projects Select projects in 2017 and 2018 Contractor request for using the T2, no cost supplemental Starting in 2019 (Special Provision) all projects Concrete overlay of an existing asphalt pavement Concrete placed directly on the grade Not used on UBOL Due to steel in the existing concrete
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Minnesota DOT Frequency of Testing Prior to 2014
Coring frequency was 1 per 1000 lineal ft per lane 2014 to current (reduced coring and implemented more probing) 1 probe per 1000 lineal ft per lane 1 verification core per 4 probes 1 random core per 4000 lineal ft per lane 2019 and beyond (Special Provision) 1 scan per 1000 lineal ft per lane 1 verification core per 8 scans 1 random core per 8000 lineal ft per lane 8 cores / 8000 l ft 4 cores / 8000 l ft 2 cores / 8000 l ft Cores and scans will be used to determine final average thickness
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Wisconsin DOT Timeline Started using Thickness for Acceptance
2017 Ownership Paving Contractors
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Wisconsin DOT Procedure 250 ft basic units (for each lane)
Two plates in each unit by contractor Longitudinal and transverse locations randomly determined Within 3 feet from steel or within 4 feet from a joint Anchor plates using 16D common nail
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Wisconsin DOT Procedure The department will measure thickness at one random location in each unit If the initial measurement falls within the 80 to 50 percent pay range, the department will measure at the second plate in that unit and average the results to determine pay adjustments
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Wisconsin DOT Thickness Penalty
Pavement thinner than plan thickness by Percent of the Contract Unit Price > 0.25 in but ≤ 0.5 inch 80 > 0.5 in but ≤ 0.75 inch 60 > 0.75 in but ≤ 1 inch 50 > 1 inch unacceptable The department adjusts pay based on the average of 2 measurements per unit
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What about Asphalt Pavements?
Tonnage (asphalt) versus SqYards (concrete) Cores for density purposes (asphalt) Minnesota DOT Experience West Central district uses a pay item of square yard inch (SYI) for asphalt Personnel Issue Weight tickets - Fewer agency folks in the field Additional cores are taken (above the normal density cores) to verify pavement thickness
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Asphalt Pavements Minnesota DOT Experience
Procedure Targets placement Immediately prior to paving Discs placed on the base Invisible after the first lift (randomness) 4.7” plates, secured with one nail Number of plates based on the total tonnage Vary based on pavement thickness Graduated deduction if paved too thin
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State of Practice Full-Fledged Implementation Iowa DOT Wisconsin DOT
Minnesota DOT West Virginia DOT Illinois Tollway Alabama DOT Nevada DOT Pennsylvania DOT Certain districts
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Field trials already performed or underway
Planned Activities Implementing in 2019 Washington State DOT North Dakota DOT Considering implementing in 2019 or beyond Delaware DOT Florida DOT Indiana DOT North Carolina DOT Arizona DOT Field trials already performed or underway
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Contractors Perspective
Need not wait until the cores are cut for payment Cutting fewer or no cores In case of disputes, core is taken over the metal plate The scanner has been shown to be accurate.
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Summary Based on the State’s Data
Excellent correlation to core test results Repeatable between operators and equipment Can work for both concrete and asphalt pavements Proven technology for Quality Control and Agency Acceptance Major Benefits Better statistical data (agency) Almost real-time testing (contractor) Cost, time, personnel (agency and contractor)
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